netem: fix possible NULL deref in netem_dequeue()

commit aec0a40a6f7884 ("netem: use rb tree to implement the time queue")
added a regression if a child qdisc is attached to netem, as we perform
a NULL dereference.

Fix this by adding a temporary variable to cache
netem_skb_cb(skb)->time_to_send.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 3 deletions

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index ed0082cf8eff..82f6016d89ab 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -554,10 +554,13 @@ deliver:
         }
         p = rb_first(&q->t_root);
         if (p) {
+                psched_time_t time_to_send;
+
                 skb = netem_rb_to_skb(p);
 
                 /* if more time remaining? */
-                if (netem_skb_cb(skb)->time_to_send <= psched_get_time()) {
+                time_to_send = netem_skb_cb(skb)->time_to_send;
+                if (time_to_send <= psched_get_time()) {
                         rb_erase(p, &q->t_root);
 
                         sch->q.qlen--;
@@ -593,8 +596,7 @@ deliver:
                         if (skb)
                                 goto deliver;
                 }
-                qdisc_watchdog_schedule(&q->watchdog,
-                                        netem_skb_cb(skb)->time_to_send);
+                qdisc_watchdog_schedule(&q->watchdog, time_to_send);
         }
 
         if (q->qdisc) {